In the prior art, various compact and closed press sections are known, in which there is a combination of press rolls, whose press rolls form, as a rule, at least three press nips with one another. Of these prior art press sections, reference should be made to the assignee's "Sym-Press II" press section and the assignee's "Sym-Press O" press section (""=trade marks).
In a typical "Sym-Press II" press section, the first twin-wire nip is formed between a hollow-faced lower press roll and an upper press-suction roll. The suction roll forms the second nip with the smooth-faced center roll of the press, and the third press nip is also formed in connection with the center roll. Further, this press section may include a separate nip, as a rule a fourth nip in the press, after which the web is transferred to the subsequent drying section.
In a typical "Sym-Press O" press section, the first twin-felt horizontal nip is formed between a lower hollow-faced roll and an upper press-suction roll, after which the web follows the upper fabric as a vertical run into the second nip, which is formed between a hollow-faced roll separate from the rolls in the first nip and the smooth-faced center roll. In connection with the center roll, a third press nip, which is provided with a felt, is formed by means of a hollow-faced press roll.
In the manner known in prior art, the web is detached from the smooth face of the center roll in the press section by means of a pulling force parallel to the running direction of the web so that the web will be passed in a free, unsupported draw from the smooth-faced center roll onward. This free draw is quite critical in view of the operation of a paper machine. In this free draw, to produce the detaching force, a speed difference is employed which stretches the web causing certain significant drawbacks. Moreover, this unsupported draw forms a problematic point for the web rendering it susceptible to breaks and stoppages of the paper machine.
In general, the prior art has not offered efficient means for controlling the detaching of the web from the smooth-faced center roll and possibly to be passed in a subsequent free draw. The unfavorable properties of granite, which is commonly used as the material of the center roll, have for their part, also restricted the control of the detaching of the web and control of the web running in the free draw.
With ever increasing running speeds of paper machines, the free draw of the web has become an ever more important problem point. In the near future, paper machine running speeds of an order of as high as about 2000 m/min are contemplated. The open draw of the web from a smooth-faced roll in the press forms or will form a bottleneck which for its part prevents increased running speeds of paper machines.
It is a further problem that, in a paper machine, different paper qualities are often produced, whose adhesion to the smooth face of the center roll is different, which results in variations in the necessary detaching tension of the web to be provided in the conventional web-detaching arrangement mentioned above.
In a "Sym-Press II" press section, in the second and third press nips formed in conjunction with the center roll, the surface properties of the center roll must be such that the moist web adheres to the roll face as well as possible. On the other hand, the web should be detached from the roll face quite readily for transfer to the subsequent drying section. The fulfillment of these paradoxical requirements has not been too successful in every respect by the prior art arrangements.
In the prior art, some transfer-suction rolls have been suggested which have been meant to form a transfer nip against the smooth-faced center roll in the press section. It is a feature common to these prior art transfer-suction rolls that they apply only suction to the transfer zone defined between the transfer-suction roll and the center roll and do not operate in the intended way in practice. This is due to the fact that, by means of the prior art transfer-suction rolls, it has not been possible to separate the paper web from the center roll by suction alone, because, at the transfer point, the suction force causes the web to adhere to the smooth face of the center roll tightly and, as a rule, even by the intermediate of a water film. This effect is caused in part by the porosity of the fabric and the web. Thus, the negative pressure applied to the web in the suction zone of the transfer-suction roll produces a corresponding negative pressure (partial vacuum) between the web and the smooth face of the center roll. Consequently, the web is not detached from the smooth face of the center roll but rather the partial vacuum between the web and the smooth face of the center roll causes the web to follow the smooth face of the center roll, i.e., detachment does not occur. Owing to the above, one has been content with considering it almost as a law of nature that the web cannot be detached from a smooth roll face by suction alone.
With respect to the prior art related to the invention, reference is also made to the assignee's Finnish Patent Application Nos. 870308 and 870309, which correspond to U.S. Pat. Nos. 4,919,759 and 4,889,598, respectively, which are incorporated by reference herein.
In FI 870308, a method and a device are described for detaching of the web by means of an unsupported draw from a smooth-faced press roll. In this method, the temperature of the press roll is regulated and, by means of this regulation, the adhesion between the roll face and the paper web to be detached is affected, and thereby the detaching angle and/or the detaching tension of the paper web is set within an optimal range.
In FI 870309, a method and a device are described in which the web is detached from a smooth roll face as an unsupported draw and, in the area of the detaching point, a momentary and local induction heating effect is applied to the web from outside the smooth-faced press roll, by means of which heating effect the water present between the web and the roll face is heated, preferably vaporized, locally within the area of the detaching point to detach the web from the roll face.
It is important to note that FI 870309 does not describe the use of a transfer-suction zone in which the prevailing pressure is lower than atmospheric pressure, i.e., in which negative pressure is applied, or a transfer-suction zone in which suction is applied before the web is transferred from the center roll. Indeed, as shown in FIG. 2 therein, negative pressure is applied not even at the detaching point but only after the detaching point, i.e., after the web has already been transferred from the roll, so that the web remains on the drying wire in a straight run from the center roll to the suction zone of the leading roll. Blowings produce a positive pressure and not a negative pressure at the heating device and at the detaching point. Thus, FI 870309 does not use suction in a transfer zone before the web has separated from the smooth roll face in conjunction with the application of heat in the transfer zone.
It is also important to note that in FI 870309, electromagnetic inductive heating is applied to heat the web. Therefore, the requirement that the center roll include conductive material limits the selection of materials for the center roll.
In summary, in the prior art, there are press sections which include means for applying suction through a transfer-suction roll to attempt to detach a web from a smooth mantle face, which are unsuccessful in view of the fact that the suction unavoidably increases the adherence of the web to the smooth mantle face as noted above, but do not describe the specific application of heat to the web and/or mantle face to aid in the detachment of the web from the smooth mantle face in conjunction with the application of suction. Also, there are press sections which include means for applying heat, albeit a local and momentary heating effect, to a roll to detach a web therefrom, e.g., FI 870309, but do not describe the application of suction prior to the detachment of the web from the mantle face. However, there is no suggestion in the prior art to combine the application of suction prior to detachment of a web from a smooth mantle face and the application of heat in a specific relationship to provide for efficient detachment of the web from the smooth mantle face.
In the following, typical tensions that occur in a free draw of the web will be examined in the light of an example.
In a free draw of any web-like material, the tension arises, wherein
F=tension [N/m] PA1 Q=grammage [kg/m.sup.2 ] PA1 v=velocity [m/s]
Since the web has a certain finite breaking strength F.sub.M, the formula for the limit speed is ##EQU1##
Typically, the wet breaking strength of newsprint (F.sub.M) is about 200 N/m, and the corresponding grammage is about 42 g/m.sup.2, and the moisture content in the first free gap in the nip is about 42%. ##EQU2##
On the other hand, the typical strength values are average values, but the web breaks at the weakest point. If there is an occasional, local strength of about 50 N/m in the web, the following velocity is obtained: ##EQU3##
The above velocity v.sub.limit is at present a typical running speed employed in paper machines for which reasons the web breaks now and then at the open draw.
If it is desirable to increase the speed of a paper machine, the web must be made generally stronger or more homogeneous (no weak points), or the free gaps in the run of the web in the press section must be eliminated. The present invention endeavors to solve these problems by means of elimination of the free gaps.